1. Field of the Invention
This invention relates to a method for producing single wall carbon nanotubes, also known as linear fullerenes, employing unsupported metal containing catalysts, for decomposition of a C.sub.1 to C.sub.6 carbon feedstock such as carbon monoxide.
2. Description of the Related Art
Multi-walled Carbon Nanotubes
Multi-walled carbon nanotubes, or fibrils, are well-known. Typically, carbon fibrils have a core region comprising a series of graphitic layers of carbon.
Since the 1970's, carbon nanotubes and fibrils have been identified as materials of interest for a variety of applications. Submicron graphitic fibrils belong to a class of materials sometimes called vapor grown carbon fibers. Carbon fibrils are vermicular carbon deposits having diameters less than approximately 1.0.mu.. They exist in a variety of forms and have been prepared through the catalytic decomposition of various carbon-containing gases at metal surfaces. Such vermicular carbon deposits have been observed almost since the advent of electron microscopy. A good early survey and reference is found in Baker and Harris, Chemistry and Physics of Carbon, Walker and Thrower ed., Vol. 14, 1978, p. 83, and in Rodriguez, N., J. Mater. Research, Vol. 8, p. 3233 (1993).
Carbon fibrils were seen to originate from a metal catalyst particle which, in the presence of a hydrocarbon containing gas, became supersaturated in carbon. A cylindrical ordered graphitic core is extruded which immediately became coated with an outer layer of pyrolytically deposited graphite. These fibrils with a pyrolytic overcoat typically have diameters in excess of 0.mu.. (Oberlin, A. and Endo, M., J. Crystal Growth, 32:335-349(1976).)
Tibbetts has described the formation of straight carbon fibers through pyrolysis of natural gas at temperatures of 950.degree.-1075.degree. C., Appl. Phys. Lett. 42(8):666(18.backslash.983). The fibers are reported to grow in two stages where the fibers first lengthen catalytically and then thicken by pyrolytic deposition of carbon. Tibbetts reports that these stages are "overlapping", and is unable to grow filaments free of pyrolytically deposited carbon. In addition, Tibbett's approach is commercially impracticable for at least two reasons. First, initiation of fiber growth occurs only after slow carbonization of the steel tube (typically about ten hours), leading to a low overall rate of fiber production. Second, the reaction tube is consumed in the fiber forming process, making commercial scale-up difficult and expensive.
In 1983, Tennent, U.S. Pat. No. 4,663,230 succeeded in growing cylindrical ordered graphite cores, uncontaminated with pyrolytic carbon, resulting in smaller diameter fibrils, typically 35 to 700 .ANG. (0.0035 to 0.070.mu.), and an ordered "as grown" graphitic surface. Tennent '230 describes carbon fibrils free of a continuous thermal carbon overcoat and having multiple graphitic outer layers that are substantially parallel to the fibril axis. They may be characterized as having their c-axes, (the axes which are perpendicular to the tangents of the curved layers of graphite) substantially perpendicular to their cylindrical axes, and having diameters no greater than 0.1.mu. and length to diameter ratios of at least 5.
Tennent, et al., U.S. Pat. No. 5,171,560 describes carbon fibrils free of thermal overcoat and having graphitic layers substantially parallel to the fibril axes such that the projection of said layers on said fibril axes extends for a distance of at least two fibril diameters. Typically, such fibrils are substantially cylindrical, graphitic nanotubes of substantially constant diameter and comprise cylindrical graphitic sheets whose c-axes are substantially perpendicular to their cylindrical axis. They are substantially free of pyrolytically deposited carbon, have a diameter less than 0.1.mu. and a length to diameter ratio of greater than 5.
Moy et al., U.S. Ser. No. 07/887,307 filed May 22, 1992, describes fibrils prepared as aggregates having various macroscopic morphologies (as determined by scanning electron microscopy) including morphologies resembling bird nests ("BN"), combed yarn ("CY") or "open net" ("ON") structures.
Multi-walled carbon nanotubes of a morphology similar to the catalytically grown fibrils described above have been grown in a high temperature carbon arc (Iijima, Nature 354 56 1991). (Iijima also describes in a later publication arc-grown single-walled nanotubes having only a single layer of carbon arranged in the form of a linear Fullerene.) It is now generally accepted (Weaver, Science 265 1994) that these arc-grown nanofibers have the same morphology as the earlier catalytically grown fibrils of Tennent.
Single-walled Carbon Nanotubes
As mentioned above, the Iijima method partially results in single-walled nanotubes, i.e., nanotubes having only a single layer of carbon arranged in the form of a linear Fullerene.
U.S. Pat. No. 5,424,054 to Bethune et al. describes a process for producing single-walled carbon nanotubes by contacting carbon vapor with cobalt catalyst. The carbon vapor is produced by electric arc heating of solid carbon, which can be amorphous carbon, graphite, activated or decolorizing carbon or mixtures thereof. Other techniques of carbon heating are discussed, for instance laser heating, electron beam heating and RF induction heating.
Smalley (Guo, T., Nikoleev, P., Thess, A., Colbert, D. T., and Smally, R. E., Chem. Phys. Lett. 243: 1-12 (1995)) describes a method of producing single-walled carbon nanotubes wherein graphite rods and a transition metal are simultaneously vaporized by a high-temperature laser.
Smalley (Thess, A., Lee, R., Nikolaev, P., Dai, H., Petit, P., Robert, J., Xu, C., Lee, Y. H., Kim, S. G., Rinzler, A. G., Colbert, D. T., Scuseria, G. E., Tonarek, D., Fischer, J. E., and Smalley, R. E., Science, 273: 483-487 (1996)) also describes a process for production of single-walled carbon nanotubes in which a graphite rod containing a small amount of transition metal is laser vaporized in an oven at about 1200.degree. C. Single-wall nanotubes were reported to be produced in yields of more than 70%.
Each of the techniques described above employs solid carbon as the carbon feedstock. These techniques are inherently disadvantageous. Specifically, solid carbon vaporization via electric arc or laser apparatus is costly and difficult to operate on the commercial or industrial scale.
Supported metal catalysts for formation of SWNT are also known. Smalley (Dai., H., Rinzler, A. G., Nikolaev, P., Thess, A., Colbert, D. T., and Smalley, R. E., Chem. Phys. Lett. 260: 471-475 (1996)) describes supported Co, Ni and Mo catalysts for growth of both multi-walled nanotubes and single-walled nanotubes from CO, and a proposed mechanism for their formation.
However, supported metal catalysts are inherently disadvantageous, as the support is necessarily incorporated into the single-walled carbon nanotube formed therefrom. Single-walled nanotubes contaminated with the support material are obviously less desirable compared to single-walled nanotubes not having such contamination.